1. Field of the Invention
The present invention generally relates to gain control in a phase lock loop, and more specifically to phase lock loop gain control using scaled unit current sources.
2. Background Art
Radio frequency (RF) transmitters and receivers perform frequency translation by mixing an input signal with a local oscillator (LO) signal.
Preferably, the LO signal should have a frequency spectrum that is as close to a pure tone as possible in order to maximize system performance during the signal mixing operation. The deviation of the LO signal from a pure tone is quantified as phase noise or phase jitter, and is generally referred to as spectral purity. In other words, a LO signal with good spectral purity has low phase noise.
Phase-locked loops (PLLs) are often used in frequency synthesizers to generate the LO signal. A PLL frequency synthesizer produces an output signal, typically a sinewave or square wave, that is a frequency multiple of an input reference signal. The PLL output signal is also in phase synchronization with the input reference signal. PLLs are feedback loops, and therefore are susceptible to instability. Therefore, loop stability is a key performance parameter for PLLs, in addition to spectral purity of the output signal.
A resonant-tuned voltage controlled oscillator (VCO) is typically utilized in a PLL to generate the PLL output signal. A resonant tuned VCO includes an active device and a resonant LC circuit, where the impedance of the resonant LC circuit becomes a short or an open at a resonant frequency. When the resonant circuit is connected in parallel with the active device, a positive feedback path is created in the active device at the resonant frequency of the LC circuit. The positive feedback path causes the active device to oscillate at the resonant frequency of the LC circuit.
The resonant tuned LC circuit typically includes multiple fixed capacitors that can be switched in or out of the LC circuit, a varactor diode, and at least one inductor. The resonant frequency of the LC circuit (and therefore the oscillation frequency of the VCO) is tuned via a coarse tuning mechanism and a fine tuning mechanism Coarse frequency tuning (or band-selection) is performed by switching one or more of the fixed capacitors in the LC circuit. Whereas, fine frequency tuning is performed by changing the voltage across the varactor diode, which produces a capacitance that varies depending on the applied tuning voltage. Both tuning mechanisms operate by changing the capacitance, and therefore the resonant frequency of the LC circuit. The varactor tuning range is slightly larger than one fixed capacitor, and therefore provides some overlap between the fixed capacitors.
VCO gain is defined as the VCO frequency shift per unit change in the varactor tuning voltage. A problem with varactor-tuned VCOs is that the VCO gain verses fixed capacitance is variable. In other words, the VCO frequency shift verses tuning voltage is dependent on the fixed capacitance that is switched-in to the LC circuit. The variable VCO gain creates difficulties when designing a PLL because the entire PLL loop gain, bandwidth, and damping response varies with respect to the oscillator frequency. This in turn makes it difficult to optimize the output phase noise and reduces overall spectral purity. Therefore, it is desirable to compensate for the variable VCO gain, in order to maintain the overall PLL gain at a desired optimum value.
In addition to the VCO gain, it is desirable to adjust or tune other PLL characteristics, such as loop bandwidth, reference frequency, and damping factor, without having to tune or replace PLL components.
The gain compensator invention compensates for gain variation in a varactor-tuned VCO in order to maintain the overall PLL gain at a desired level over frequency. The VCO includes a LC circuit that has multiple fixed capacitors that are arranged in parallel with the varactor diode and the active portion of the VCO. The fixed capacitors are switched-in to the LC circuit by corresponding capacitor control signals. Coarse frequency tuning (also called band-select tuning) is performed by adding or subtracting one or more of the fixed capacitors to the LC circuit according to the capacitor control signal. Fine frequency tuning is performed by adjusting the tuning voltage on the varactor diode, where the VCO gain is defined as the frequency shift per unit change in varactor tuning voltage. VCO gain varies with the fixed capacitance that is switched-in to the LC circuit, and therefore changes with band-select tuning of the VCO. The gain compensator compensates for the variable VCO gain by generating a reference charge pump current for the PLL based on information that is carried in the capacitor control signal. Therefore, the gain compensator is able to simultaneously adjust the charge pump current to maintain an overall flat PLL gain as fixed capacitors are incrementally added to (or subtracted from) the LC circuit.
The gain compensator includes one or more cells that each correspond to a particular VCO that can be switched into the PLL at a given time. A VCO control signal selects a particular VCO for the PLL based on frequency, and also activates the appropriate cell. Each cell includes a plurality of unit current sources, where each unit current source substantially replicates (or copies) a pre-defined reference scale current. The unit current sources are arranged into one or more groups, where each group corresponds to a fixed capacitor in the LC circuit. Each group of unit current generates a portion of the total pump current when the corresponding capacitor is switched-in to the LC circuit. The number of unit current sources in each group is determined to compensate for the variable VCO gain that occurs when the corresponding fixed capacitor is switched-in to the LC circuit. Each group of unit current sources is activated by the same capacitor control signal that controls the corresponding fixed capacitor. Therefore, when a fixed capacitor is switched-in to the LC circuit, the corresponding group of unit current sources is simultaneously activated and switched-in to the cell to compensate for the variable VCO gain that is caused by the fixed capacitor.
An advantage of the gain compensator invention is that the number of unit current sources that are activated for a corresponding fixed capacitor is arbitrary, but the current produced is linearly proportional to the reference scale current. In other words, there is no predefined relationship between the number of unit current sources in each group that would restrict the relative amount of current produced by each group. Therefore, the total pump current can be freely optimized to incrementally adjust for the variable VCO gain that is associated with various combinations of fixed capacitors.
A further advantage of the gain compensator invention is that the reference scale current for the gain compensator cells is generated based on a PLL control signal. The PLL control signal specifics various PLL characteristics, such as the frequency of the reference signal, the PLL bandwidth, and the PLL damping factor, etc. Since the unit current sources are configured to replicate the reference scale current, all of the unit current sources can be simultaneously adjusted by changing the reference scale current. Therefore, the charge pump current can be efficiently adjusted to tune the mentioned characteristics of PLL for different operating conditions, without requiring the replacement of PLL components.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.